RFC 5445

Basic Forward Error Correction (FEC) Schemes

Network Working Group M. Watson
Request for Comments: 5445 Digital Fountain
Obsoletes: 3452, 3695 March 2009
Category: Standards Track
Basic Forward Error Correction (FEC) Schemes
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
This document provides Forward Error Correction (FEC) Scheme
specifications according to the Reliable Multicast Transport (RMT)
FEC building block for the Compact No-Code FEC Scheme, the Small
Block, Large Block, and Expandable FEC Scheme, the Small Block
Systematic FEC Scheme, and the Compact FEC Scheme. This document
obsoletes RFC 3695 and assumes responsibility for the FEC Schemes
defined in RFC 3452.

1. Introduction
The document specifies the following FEC Schemes according to the
specification requirements of the FEC building block [RFC5052]:
o Compact No-Code FEC Scheme
o Small Block, Large Block, and Expandable FEC Scheme
o Small Block Systematic FEC Scheme
o Compact FEC Scheme
This document inherits the context, language, declarations and
restrictions of the FEC building block [RFC5052]. This document also
uses the terminology of the companion document [RFC3453], which
describes the use of FEC codes within the context of reliable IP
multicast transport and provides an introduction to some commonly
used FEC codes.
Building blocks are defined in [RFC3048]. This document follows the
general guidelines provided in [RFC3269].
[RFC3452] and [RFC3695] contain previous versions of the FEC Schemes
defined in this specification. These RFCs were published in the
"Experimental" category. It was the stated intent of the RMT working
group to re-submit these specifications as an IETF Proposed Standard
in due course. This document obsoletes [RFC3695]. [RFC3452] has
already been obsoleted by [RFC5052], and this document assumes
responsibility for aspects of [RFC3452] that were not included in
[RFC5052].
This Proposed Standard specification is thus based on and backwards
compatible with the FEC Schemes defined in [RFC3452] and [RFC3695],
updated according to accumulated experience and growing protocol
maturity since their original publication. Said experience applies
both to this specification itself and to congestion control
strategies related to the use of this specification.
The differences between the FEC Scheme specifications in [RFC3452]
and [RFC3695] and this document are listed in Section 10.
Integer fields specified in this document are all encoded in network
byte order.

2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Compact No-Code FEC Scheme
3.1. Introduction
The Compact No-code FEC Scheme is a Fully-Specified FEC Scheme. The
scheme requires no FEC coding and is specified primarily to allow
simple interoperability testing between different implementations of
protocol instantiations that use the FEC building block.
3.2. Formats and Codes
3.2.1. FEC Payload ID(s)
The FEC Payload ID for the Compact No-Code FEC Scheme is composed of
a Source Block Number and an Encoding Symbol ID as shown in Figure 1.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Block Number | Encoding Symbol ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: FEC Payload ID Format for Compact No-Code FEC Scheme
The Source Block Number (SBN) is a 16-bit unsigned integer that is
used to identify from which source block of the object the encoding
symbol in the payload of the packet is generated. There are two
possible modes: in the unique SBN mode, each source block within the
object has a unique Source Block Number associated with it, and in
the non-unique SBN mode, the same Source Block Number may be used for
more than one source block within the object. Which mode is being
used for an object is outside the scope of this document and MUST be
communicated, either explicitly or implicitly, out-of-band to
receivers.
If the unique SBN mode is used, then successive Source Block Numbers
are associated with consecutive source blocks of the object starting
with Source Block Number 0 for the first source block of the object.
In this case, there are at most 2^^16 source blocks in the object.

If the non-unique SBN mode is used, then the mapping from source
blocks to Source Block Numbers MUST be communicated out-of-band to
receivers, and how this is done is outside the scope of this
document. This mapping could be implicit, for example, determined by
the transmission order of the source blocks. In non-unique SBN mode,
packets for two different source blocks mapped to the same Source
Block Number SHOULD NOT be sent within an interval of time that is
shorter than the transport time of a source block. The transport
time of a source block includes the amount of time needed to process
the source block at the sender transport layer, the network transit
time for packets, and the amount of time needed to process the source
block at the receiver transport. This allows the receiver to clearly
decide which packets belong to which source block.
The Encoding Symbol ID is a 16-bit unsigned integer that identifies
which specific encoding symbol generated from the source block is
carried in the packet payload. The exact details of the
correspondence between Encoding Symbol IDs and the encoding symbols
in the packet payload are specified in Section 3.4.
3.2.2. FEC Object Transmission Information
3.2.2.1. Mandatory
The mandatory FEC Object Transmission Information element for the
Compact No-Code FEC Scheme is:
o FEC Encoding ID: zero (0)
3.2.2.2. Common
The Common FEC Object Transmission Information elements and their
value ranges for the Compact No-Code FEC Scheme are:
Transfer-Length: a non-negative integer, less than 2^^48, indicating
the length of the object in octets.
Encoding-Symbol-Length: a non-negative integer, less than 2^^16,
indicating the length of each encoding symbol in octets.
Maximum-Source-Block-Length: a non-negative integer, less than
2^^32, indicating the maximum number of source symbols in a source
block.
The encoded Common FEC Object Transmission Information is defined in
Figure 2.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transfer Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol Length | Max. Source Block Length (MSB)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max. Source Block Length (LSB)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Encoded Common FEC Object Transmission Information (OTI)
for Compact No-Code FEC Scheme
The Transfer Length, Encoding Symbol Length, and Maximum Source Block
Length are encoded as unsigned integers, of length 48 bits, 16 bits,
and 32 bits, respectively.
All Encoding Symbols of a transport object MUST have length equal to
the length specified in the Encoding Symbol Length element, with the
optional exception of the last source symbol of the last source block
(so that redundant padding is not mandatory in this last symbol).
This last source symbol MUST be logically padded out with zeroes when
another Encoding Symbol is computed based on this source symbol to
ensure the same interpretation of this Encoding Symbol value by the
sender and receiver. However, this padding does not actually need to
be sent with the data of the last source symbol.
The "Reserved" field in the Encoded FEC Object Transmission
Information MUST be set to zero by senders and its value MUST be
ignored by receivers.
Note: this FEC Scheme was first defined in [RFC3695], which did
not require that the Encoding Symbol Length should be the same for
every source block. This document introduces a general
requirement that the Encoding Symbol Length be the same across
source blocks. Since no protocols were defined that support
variation in the Encoding Symbol Length between source blocks,
this can be done without introducing backwards compatibility
issues.
3.2.2.3. Scheme-Specific
No Scheme-Specific FEC Object Transmission Information elements are
defined by this FEC Scheme.

3.3. Procedures
The algorithm defined in Section 9.1. of [RFC5052] MUST be used to
partition the file into source blocks.
3.4. FEC Code Specification
The Compact No-Code FEC Scheme does not require FEC encoding or
decoding. Instead, each encoding symbol consists of consecutive
bytes of a source block of the object.
The following two subsections describe the details of how the Compact
No-Code FEC Scheme operates for each source block of an object.
3.4.1. Source Block Logistics
Let X > 0 be the length of a source block in bytes. Let L > 0 be the
length of the encoding symbol contained in the payload of each
packet. The value of X and L are part of the FEC Object Transmission
Information, and how this information is communicated to a receiver
is outside the scope of this document.
For a given source block X bytes in length with Source Block Number
I, let N = X/L rounded up to the nearest integer. The encoding
symbol carried in the payload of a packet consists of a consecutive
portion of the source block. The source block is logically
partitioned into N encoding symbols, each L bytes in length, and the
corresponding Encoding Symbol IDs range from 0 through N-1 starting
at the beginning of the source block and proceeding to the end.
Thus, the encoding symbol with Encoding Symbol ID Y consists of bytes
L*Y through L*(Y+1)-1 of the source block, where the bytes of the
source block are numbered from 0 through X-1. If X/L is not integral
then the last encoding symbol with Encoding Symbol ID = N-1 consists
of bytes L*(N-1) through the last byte X-1 of the source block, and
the remaining L*N - X bytes of the encoding symbol can by padded out
with zeroes.
As an example, suppose that the source block length X = 20,400 and
encoding symbol length L = 1,000. The encoding symbol with Encoding
Symbol ID = 10 contains bytes 10,000 through 10,999 of the source
block, and the encoding symbol with Encoding Symbol ID = 20 contains
bytes 20,000 through the last byte 20,399 of the source block and the
remaining 600 bytes of the encoding symbol can be padded with zeroes.
There are no restrictions beyond the rules stated above on how a
sender generates encoding symbols to send from a source block.
However, it is recommended that an implementor refer to the companion
document [RFC3452] for general advice.

In the next subsection, a procedure is recommended for sending and
receiving source blocks.
3.4.2. Sending and Receiving a Source Block
The following carousel procedure is RECOMMENDED for a sender to
generate packets containing FEC Payload IDs and corresponding
encoding symbols for a source block with Source Block Number I. Set
the length in bytes of an encoding symbol to a fixed value L, which
is reasonable for a packet payload (e.g., ensure that the total
packet size does not exceed the MTU) and that is smaller than the
source block length X, e.g., L = 1,000 for X >= 1,000. Initialize Y
to a value randomly chosen in the interval [0..N-1]. Repeat the
following for each packet of the source block to be sent.
o If Y <= N-1, then generate the encoding symbol Y.
o Within the FEC Payload ID, set the Source Block Length to X, set
the Source Block Number = I, set the Encoding Symbol ID = Y, place
the FEC Payload ID and the encoding symbol into the packet to
send.
o In preparation for the generation of the next packet: if Y < N-1
then increment Y by one else if Y = N-1 then reset Y to zero.
The following procedure is RECOMMENDED for a receiver to recover the
source block based on receiving packets for the source block from a
sender that is using the carousel procedure described above. The
receiver can determine from which source block a received packet was
generated by the Source Block Number carried in the FEC Payload ID.
Upon receipt of the first FEC Payload ID for a source block, the
receiver uses the Source Block Length and Encoding Symbol Length
received out-of-band as part of the FEC Object Transmission
Information to determine the length X in bytes of the source block
and length L in bytes of each encoding symbol. The receiver
allocates space for the X bytes that the source block requires. The
receiver also computes the length of the encoding symbol in the
payload of the packet by subtracting the packet header length from
the total length of the received packet. The receiver checks that
this symbol length is equal to L, except in the case that this is the
last symbol of the source block in which case the symbol length in
the packet may be less than L. After calculating N = X/L rounded up
to the nearest integer, the receiver allocates a boolean array
RECEIVED[0..N-1] with all N entries initialized to false to track
received encoding symbols. The receiver keeps receiving packets for
the source block as long as there is at least one entry in RECEIVED
still set to false or until the application decides to give up on
this source block and move on to other source blocks. For each

received packet for the source block (including the first packet),
the steps to be taken to help recover the source block are as
follows. Let Y be the value of the Encoding Symbol ID within the FEC
Payload ID of the packet. If Y <= N-1, then the receiver copies the
encoding symbol into the appropriate place within the space reserved
for the source block and sets RECEIVED[Y] = true. If all N entries
of RECEIVED are true, then the receiver has recovered the entire
source block.
4. Small Block, Large Block, and Expandable FEC Scheme
4.1. Introduction
This section defines an Under-Specified FEC Scheme for Small Block
FEC codes, Large Block FEC codes, and Expandable FEC codes as
described in [RFC3453].
4.2. Formats and Codes
4.2.1. FEC Payload ID(s)
The FEC Payload ID is composed of a Source Block Number and an
Encoding Symbol ID structured as shown in Figure 3.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Block Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: FEC Payload ID Format for Small Block, Large Block, and
Expandable FEC Codes
The Source Block Number is a 32-bit unsigned integer that identifies
from which source block of the object the encoding symbol(s) in the
payload are generated. These blocks are numbered consecutively from
0 to N-1, where N is the number of source blocks in the object.
The Encoding Symbol ID is a 32-bit unsigned integer that identifies
which specific encoding symbol(s) generated from the source block are
carried in the packet payload. The exact details of the
correspondence between Encoding Symbol IDs and the encoding symbol(s)
in the packet payload are dependent on the particular FEC Scheme
instance used as identified by the FEC Encoding ID and by the FEC
Instance ID, and these details may be proprietary.

4.4. FEC Code Specification
The FEC code specification and the correspondence of Encoding Symbols
IDs to encoding symbols are defined by specific instances of this
scheme and so are out of scope of this document.
5. Small Block Systematic FEC Scheme
5.1. Introduction
This section defines an Under-Specified FEC Scheme for Small Block
Systematic FEC codes as described in [RFC3453]. For Small Block
Systematic FEC codes, each source block is of length at most 65535
source symbols.
Although these codes can generally be accommodated by the FEC
Encoding ID described in Section 4, a specific FEC Encoding ID is
defined for Small Block Systematic FEC codes to allow more
flexibility and to retain header compactness. The small source block
length and small expansion factor that often characterize systematic
codes may require the data source to frequently change the source
block length. To allow the dynamic variation of the source block
length and to communicate it to the receivers with low overhead, the
block length is included in the FEC Payload ID.
5.2. Formats and Codes
5.2.1. FEC Payload ID(s)
The FEC Payload ID is composed of the Source Block Number, Source
Block Length, and the Encoding Symbol ID structured as shown in
Figure 6.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Block Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Block Length | Encoding Symbol ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: FEC Payload ID Format for Small Block Systematic FEC Scheme
The Source Block Number is a 32-bit unsigned integer that identifies
from which source block of the object the encoding symbol(s) in the
payload are generated. These blocks are numbered consecutively from
0 to N-1, where N is the number of source blocks in the object.

The Source Block Length is a 16-bit unsigned integer that specifies
the length in units of source symbols of the source block identified
by the Source Block Number.
The Encoding Symbol ID is a 16-bit unsigned integer that identifies
which specific encoding symbol(s) generated from the source block are
carried in the packet payload. Each encoding symbol is either an
original source symbol or a redundant symbol generated by the
encoder. The exact details of the correspondence between Encoding
Symbol IDs and the encoding symbol(s) in the packet payload are
dependent on the particular FEC Scheme instance used as identified by
the FEC Instance ID, and these details may be proprietary.
5.2.2. FEC Object Transmission Information
5.2.2.1. Mandatory
The mandatory FEC Object Transmission Information element for the
Small Block Systematic FEC Scheme is:
o FEC Encoding ID: 129
5.2.2.2. Common
The Common FEC Object Transmission Information elements and their
value ranges for the Small Block Systematic FEC Scheme are:
FEC Instance ID: a non-negative integer less than 2^^16.
Transfer-Length: a non-negative integer less than 2^^48, indicating
the length of the object in octets.
Encoding-Symbol-Length: a non-negative integer less than 2^^16,
indicating the length of each encoding symbol in octets.
Maximum-Source-Block-Length: a non-negative integer less than 2^^16,
indicating the maximum number of source symbols in a source block.
Max-Number-of-Encoding-Symbols: a non-negative integer less than
2^^16, indicating the maximum number of encoding symbols per block
(i.e., source plus repair symbols in the case of a systematic
code).
The encoded Common FEC Object Transmission Information is defined in
Figure 7.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transfer Length |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | FEC Instance ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol Length | Maximum Source Block Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Max. Num. of Encoding Symbols |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: FEC OTI Format for Small Block Systematic FEC Scheme
The Transfer Length (48 bits), FEC Instance ID (16 bits), Encoding
Symbol Length (16 bits), Maximum Source Block Length (16 bits), and
Maximum Number of Encoding Symbols (16 bits) are encoded as unsigned
integers.
All Encoding Symbols of a transport object MUST have length equal to
the length specified in the Encoding Symbol Length field, with the
optional exception of the last source symbol of the last source block
(so that redundant padding is not mandatory in this last symbol).
This last source symbol MUST be logically padded out with zeroes when
another Encoding Symbol is computed based on this source symbol to
ensure the same interpretation of this Encoding Symbol value by the
sender and receiver. However, this padding need not be actually sent
with the data of the last source symbol.
Note: this FEC Scheme was first defined in [RFC3452], which did
not require that the Encoding Symbol Length should be the same for
every source block. However, no protocols have been defined that
support variation in the Encoding Symbol Length between source
blocks, and thus introduction of a general requirement that the
Encoding Symbol Length be the same across source blocks (as
defined here) should not cause backwards compatibility issues and
will aid interoperability.
5.2.2.3. Scheme-Specific
The Scheme-Specific FEC Object Transmission Information format
defined in Section 4.2.2.3 SHALL be used.
5.3. Procedures
The algorithm defined in Section 9.1. of [RFC5052] MAY be used to
partition the file into source blocks. Otherwise, the FEC Scheme
instance MUST specify the algorithm that is used.

6.4. FEC Code Specification
The FEC code specification and the correspondence of Encoding Symbols
IDs to encoding symbols are defined by specific instances of this
scheme and so are out of scope of this document.
7. Security Considerations
This specification does not introduce any further security
considerations beyond those described in [RFC5052].
8. Acknowledgements
This document is substantially based on [RFC3695] by Michael Luby and
Lorenzo Vicisano and [RFC3452] by Michael Luby, Lorenzo Vicisano, Jim
Gemmell, Luigi Rizzo, Mark Handley, and Jon Crowcroft.
9. IANA Considerations
FEC Encoding IDs 0 and 130 were first defined and registered in the
ietf:rmt:fec:encoding namespace by [RFC3695]. This document updates
and obsoletes the definitions from that specification. References to
that specification should be replaced with references to this
document.
FEC Encoding IDs 128 and 129 were first defined and registered in the
ietf:rmt:fec:encoding namespace by [RFC3452]. This document updates
and obsoletes the definitions from that specification. References to
that specification should be replaced with references to this
document.
Values of FEC Encoding IDs and FEC Instance IDs are subject to IANA
registration. For general guidelines on IANA considerations as they
apply to this document, see [RFC5052].
This document assigns the Fully-Specified FEC Encoding ID 0 under the
ietf:rmt:fec:encoding name-space (which was previously assigned by
[RFC3695], which is obsoleted by this document) to "Compact No-Code"
as specified in Section 3 above.
This document assigns the Under-Specified FEC Encoding ID 128 under
the ietf:rmt:fec:encoding name-space (which was previously assigned
by [RFC3452]) to "Small Block, Large Block, and Please note that we
have added a comma between large block and expandable throughout this
document (RFC Editor style is to include a comme before the last item
of a series). If you do not object, we will ask IANA to include this
comma in their registry for consistency. --> Expandable FEC Codes" as
specified in Section 4 above.

This document assigns the Under-Specified FEC Encoding ID 129 under
the ietf:rmt:fec:encoding name-space (which was previously assigned
by [RFC3452]) to "Small Block Systematic FEC Codes" as specified in
Section 5 above.
This document assigns the Under-Specified FEC Encoding ID 130 under
the ietf:rmt:fec:encoding name-space (which was previously assigned
by [RFC3695], which is obsoleted by this document) to "Compact FEC"
as specified in Section 6 above.
As FEC Encoding IDs 128, 129, and 130 are Under-Specified, "FEC
Instance ID" sub-name-spaces must be established, in accordance to
[RFC5052]. Hence, this document also assumes responsibility for the
"FEC Instance ID" registries named.
ietf:rmt:fec:encoding:instance:128, scoped by ietf:rmt:fec:
encoding = 128
ietf:rmt:fec:encoding:instance:129, scoped by ietf:rmt:fec:
encoding = 129
ietf:rmt:fec:encoding:instance:130, scoped by ietf:rmt:fec:
encoding = 130
The values that can be assigned within these namespaces are non-
negative numeric indices. Assignment requests are granted on a
"First Come First Served" basis. [RFC5052] specifies additional
criteria that MUST be met for the assignment within the generic ietf:
rmt:fec:encoding:instance name-space. These criteria also apply to
ietf:rmt:fec:encoding:instance:128, ietf:rmt:fec:encoding:instance:
129, and ietf:rmt:fec:encoding:instance:130.
10. Changes from Schemes Defined in RFC 3452 and RFC 3695
This section describes the changes between the Experimental versions
of these FEC Scheme specifications contained in RFC 3452 [RFC3452]
and RFC 3695 [RFC3695] and those defined in this specification:
o Scheme definitions have been updated to meet the requirements of
[RFC5052].
o Complete encoding formats for the FEC Object Transmission
Information for each scheme are defined here, instead of within
content delivery protocol specifications, since the exact format
depends on the FEC Scheme.